Geological Characteristics And Genetic Mechanism Of The Chang’an Gold Deposit, Jinping, Yunnan Province

The Chang’an gold deposit is one of the largest deposits located at the southernsection of the Ailaoshan metallogenic belt. Gold mineralization is controlled bystructure (F6) significantly. The lenticular Orebody is preserved in between theSilurian and Ordovician unconformity. The boundary between orebody and wall-rockis gradual. The main ores are sulfide ores, near the surface containing part of oxideores. The ore-bearing rocks is consitituted by Ordovician Xiangyang group, SilurianKanglang groug, altered diabase and lamprophyre. Gold as micro-disseminatedstructure in the ore. There are dykes in the deposit. The ore bodies and dykes occurtogether or cut each other. The dykes and the formation have low to moderatetemperature alterations, such as silication, sericitization, carbonicacidation. Based onthe recent extensive researches and field investigations, the close symbiosis dyke withore bodies zircon LA-ICP-MS U-Pb dating shows that fine-grained syenite andsyenite porphyry dykes diagenetic age were32.5±0.1Ma and33.0±0.1Ma. Theseare basically the same with the large-scale concentrated ages of the Cenozoicalkali-rich porphyry type gold-polymetallic deposits, Ailaoshan. The studies ongeochemistry characteristics of the magmatic rocks about the Chang’an andTongchang deposits, it show that they might be sourced from the same origin. Pyritefrom Chang’an, Tongchang and Chang’anchong deposit, have been selected fordetailed study in this paper. The LA-ICP-MS trace element analyses of these pyritesdemonstrate a negative correlation between ore-forming elements and the Co/Ni ratio.The δ34S value located near0‰. The REE contents in pyrite and the alkali-richporphyry have the same features. All about these, show a close relationship betweenthe Chang’an gold deposit and Jinshajiang-Ailaoshan Himalayan alkali-rich intrusiverocks. The ore-forming fluid probably sourced from the magmatic-hydrothermalsystem, and the metamorphic water also played an important role in the mainmineralization stage. Combined with previous research results show that the Himalayan period crust-mantle interaction leads to massive alkali-rich magmaintrusion, the ore-forming fluid upward in the fracture system, extracted Au and theother metallogenetic elements from the wall rocks. Then migrated them to the properstructural spaces, and the physical-chemical condition changed rapidly. Au and theother metallogenetic elements precipitation, finally formed the gold deposit. TheHimalayan alkali-rich porphyry not only provided the impetus and energy for the orefluid, but also the main source of minerals and ore-forming fluids. Multi-phaseintrusive rock, resulting in the ore-forming fluid and mineralizing last for a long time.The mineralization age should be about32Ma.